# Recursion

 This is an AoPSWiki Word of the Week for Jan 17-23

Recursion is a method of defining something (usually a sequence or function) in terms of previously defined values. The most famous example of a recursive definition is that of the Fibonacci sequence. If we let $F_n$ be the $n$th Fibonacci number, the sequence is defined recursively by the relations $F_0 = F_1 = 1$ and $F_{n+1}=F_{n}+F_{n-1}$. (That is, each term is the sum of the previous two terms.) Then we can easily calculate early values of the sequence in terms of previous values: $F_0=1, F_1=1, F_2=2, F_3=3, F_4=5, F_5=8$, and so on.

Often, it is convenient to convert a recursive definition into a closed-form definition. For instance, the sequence defined recursively by $a_0 = 1$ and $a_n = n\cdot a_{n - 1}$ for $n > 0$ also has the closed-form definition $a_n = n!$ (where "!" represents the factorial function).